The present invention relates to a system for alleviating a select shock in automatic transmission, and more particularly to a system for alleviating a select shock taking place during engagement of a friction element after a manual selector valve has been shifted from a neutral range position to any one of a plurality of drive range positions.
An automatic transmission is known wherein a driver manually shifts a manual selector valve from a neutral range position ("N" range) to a desired one of a plurality of drive range positions corresponding to a reverse range ("R" range), an automatic drive range ("D" range), a manual second range ("II" range), and a manual first range ("I" range). More specifically, all of the friction elements (clutches and brakes) are released when the neutral range position is selected, while a selected one of the friction elements is hydraulically activated when one of the drive ranges is selected.
When the driver shifts the manual selector valve from the neutral range position to one of the drive range positions under a condition where the motor vehicle is at a standstill, a predetermined friction element is engaged in response to a hydraulic fluid pressure build-up resulting from the hydraulic fluid supplied thereto from the manual selector valve. The motor vehicle is now ready for moving from a standstill. This friction element may be called as a start-up friction element.
If, during this shift of the manual selector valve, the hydraulic fluid pressure acting on the start-up friction element increases quickly, a shock taking place upon engagement of the start-up friction element becomes great since the vehicle is at a standstill. This shock is called as a selected shock.
An automatic transmission of the RE4R01A type is known. This known transmission is manufactured in Japan by NISSAN MOTOR COMPANY LIMITED. In this known automatic transmission, a N-D accumulator is provided to control a rise in the servo activating hydraulic fluid pressure acting on a start-up friction element. A forward clutch serves as the start-up friction element in this transmission.
In controlling a back-up pressure acting on the accumulator, the magnitude of the select shock is reduced if the pressure level of the back-up pressure is kept at a sufficiently low level. However, if the pressure level is lowered enough to decrease the magnitude of the select shock to a sufficiently low level, a time lag between the instant when the manual selector valve is shifted from the neutral range position to one of the plurality of drive range positions and the subsequent instant when the start-up friction element is brought into engagement becomes long.
Explaining in detail referring to the fully drawn time charts shown in FIG. 6, when the manual selector valve is shifted from the neutral range position to the drive range position at the instant t.sub.1 under a condition where the accumulator back-up pressure P.sub.A is zero, the stroke of the accumulator piston, the hydraulic pressure P.sub.F supplied to the forward clutch for engagement of same, the transmission output torque, and the transmission input revolution speed N.sub.T vary with respect to time t after the above-mentioned instant t.sub.1 as shown by the fully drawn line curves in FIG. 6. As is readily understood from the variation of the transmission output torque, there is a great lag between the instant t.sub.1 and the completion of engagement of the forward clutch.
Thus, if the driver depresses the accelerator pedal immediately after the instant t.sub.1 for quick start of the vehicle from a standstill, the engine races during the above-mentioned time lag, inducing a substantially great shock.
An object of the present invention is to shorten the above-mentioned time lag, with the magnitude of a select shock suppressed to a sufficiently low level.